1. Field of the Invention
The present invention relates generally to integrated waveguides, and more specifically to a side-gap waveguide taper.
2. Related Art
During the recent decades, the electronics industry has seen massive expansion in the application of integrated circuit technology. As system designers were challenged with more stringent space, power and performance requirements, they turned more and more to solutions implementing integrated circuit technology. Communications systems designers were no exception to this rule. They too, continuously developed an increasing number of components in their systems using integrated circuits.
Contemporaneous with the growth of integrated circuits was the maturation of fiber-optic communications technology and semiconductor laser diode technology. As an almost direct and natural result of the natural compatibility among these technologies, the technology of integrated optics was spawned. Integrated optics, which can be defined as the integration of one or more optical guided-wave structures on a common substrate, are now used to implement numerous useful devices such as power splitters, optical switches, fiber optic transmitters and fiber optic receivers. Integrated optic devices are well suited to applications in such technologies as telecommunications, instrumentation, signal processing and sensors.
In contemporary integrated optic devices, optical channel waveguides are formed on a thin, planar, optically polished substrate. To couple light into and out of the integrated optic device, an optical fiber is butt-coupled to the device. However, differences exist between the optical fibers and the on-chip waveguides, namely, structure and material composition (i.e., differences in core size and refractive index profile). Specifically, because the difference of the refractive index between the core and cladding of a typical waveguide is higher than that of a typical fiber, the optical field is more confined in the waveguide than in the fiber. In addition, the waveguide core dimension is smaller than the fiber core dimension. Therefore, when coupling the waveguide with the fiber, there is a coupling loss. What is needed is a device to match the mode of the waveguide with the mode of the fiber. As a result, mode tapering is used for low-loss coupling of light into waveguides.
One technique used to implement mode tapering has been to change the dimensions of the waveguide. For example, see Koch et al., IEEE Photonics Technol. Lett. 2:88-90 (1990); Mahapatra and Connors, Opt. Lett. 13:169-171 (1988); and Shani et al., Appl. Phys. Lett. 55:2389-2391 (1989). However, because the integrated optic devices are manufactured using photolithographic techniques, tapering by changing both the height and the lateral dimension of the waveguide simultaneously results in a complicated fabrication process.
A second technique, proposed by Z. Weissman and A. Hardy, "2-D Mode Tapering Via Tapered Channel Waveguide Segmentation," Electronics Letters 28:1514-1516, (1992), introduces segmented waveguides to implement two-dimensional mode tapering. Modal properties of periodically segmented waveguides are analyzed by Z. Weissman and A. Hardy, "Modes of Periodically Segmented Waveguides," IEEE Journal of Lightwave Technology 11:1831-1838 (1993).
According to Weissman and Hardy, segmented waveguides are implemented by introducing a series of gaps into the waveguide such that the waveguide is segmented into a series of segments. Each segment has a gap section of length s, and a core section having a length t. A period .LAMBDA. of the segment is the sum of gap section s and core section length t.
Weissman and Hardy proposed an approach to implementing the segmented waveguide. This approach uses a fixed period segmented waveguide taper. In this approach, the period A of each segment is fixed, and gap length s is successively increased along a length of the taper.
Waveguide segmentation has also been of interest for second harmonic generation in KTP devices (Bierlein et al., Appl. Phys. Lett. 56:1725-1727 (1990); Li and Burke, Opt. Lett. 17:1195-1197 (1992)). It was found experimentally (Bierlein et al., Appl. Phys. Lett. 56:1725-1727 (1990)) that a segmented waveguide section has surprisingly good guiding properties and low radiation loss. Such surprising low-loss results were later theoretically understood based on planar wave model by Li et al. (Li and Burke, Opt. Lett. 17:1195-1197 (1992)). Their study concluded that the electromagnetic field can be described by averaging the guiding properties over the segmentation period.